In a superconducting film, the dissipationless current is carried by pairs of charge carriers. These pairs can decompose into quasiparticles, or single carrier excitations. The magnitude of the energy gap in a superconductor is determined by the density of quasiparticles. If a superconductor is perturbed, by raising its temperature, by external radiation, or by tunnel injection, the quasiparticle density increases, reducing the energy gap. Eventually the energy gap disappears and the current, now carried by normal carriers, is no longer dissipationless. As a result a voltage develops in the film. This phenomenon has been employed as the basis of a number of switching devices.
There is a class of superconductive devices in which a thin film of superconductive material is driven substantially out of equilibrium, causing a switching action. In one state the current is carried by superconducting pairs and there is no voltage across the film. In the other state, the current is carried by normal electrons and there is a voltage across the film. For proper choices of device and load resistances, the current in the superconductor in the latter state can be much smaller than in the former one, most of the current being switched to the load.
A non-equilibrium injection weak link device is described in commonly assigned U.S. Pat. No. 4,831,421, issued May 16, 1989, entitled "Superconducting Device" by W. J. Gallagher and S. I. Raider.
One difficulty which exists in all known devices of this type is the tendency of the injected excitations, or quasiparticles, to diffuse equally well in all crystal directions, due to the essentially isotropic nature of the superconductor's crystal structure. In the most promising device structures, this causes the volume of material which is perturbed to be larger than is desirable, with adverse effects on device gain and switching speed.
It has been observed that an energy gap of several high transition temperature (T.sub.c) oxide superconductors, for example La.sub.1-x Sr.sub.x CuO.sub.4 and Y.sub.1 Ba.sub.2 Cu.sub.3 O.sub.7-x, exhibits significant anisotropy. This observation is at least partially supported by an experimentally-observed difference between energy gaps obtained by infrared transmission and by tunneling measurements, and by unusually large gaps seen in tunneling measurements in oriented films along a crystal direction in which large gaps would be expected. The magnitude of the energy gap anisotropy is not presently well-defined, but indications are that the gap differs by a factor of two or more between a basal plane of the superconducting material and a direction orthogonal to the basal plane. By example, the anisotropy of Y.sub.1 Ba.sub.2 Cu.sub.3 O.sub.7-x is discussed in the following articles: Phys. Rev. Lett. Vol. 58, No. 25, pp. Lett. Vol. 59, No. 10, pp. 1160-1163, T. K. Worthington et al., 7 Sept. 1987; and Physical Review B, Vol. 36, No. 7, pp. 4025-4027, D. E. Farrell et al., 1 Sept. 1987. Gap anisotropy discussion and data is provided by J. Kirtley in an article to be published in Phys. Rev. B and in Int'l. J. Mod. Phys., 1/90.
In a commonly assigned U.S. patent application Ser. No. 07/051,552, filed May 15, 1987, "High Current Conductors and High Field Magnets Using Anisotropic Superconductors" A. Davidson et al. disclose magnets employing anisotropic superconductors.
However, until the invention described herein this observed energy gap anisotropy of high transition temperature oxide superconductors has not been exploited to provide electrical devices having the unique properties described below.
It is thus an object of the invention to provide an electrical device that operates in accordance with an energy gap anisotropy of a superconducting material of which the device is comprised.
It is another object of the invention to provide several thin film structures that operate in accordance with an energy gap anisotropy of a high transition temperature oxide superconductor of which the thin film structures are comprised.
It is a further object of the invention to provide thin film device structures in which an energy gap anisotropy of a high transition temperature oxide superconductor is exploited to control currents in the thin film device structures.